The most common and most likely answer to this crossword clue is the 12 letter word FERMENTATION
I've seen this crossword clue in Independent
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More clues leading to the result FERMENTATION
Buddy explains!
The straight part
We know that "unrest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unrest = fermentation.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FERMENTATION mean?
FERMENTATION noun- Any of many anaerobic biochemical reactions in which an enzyme (or several enzymes produced by a microorganism) catalyses the conversion of one substance into another; especially the conversion (using yeast) of sugars to alcohol or acetic acid with the evolution of carbon dioxide
- A state of agitation or excitement; a ferment.